Method and apparatus for stresstrain testing



C. M. ZENER ET AL METHOD AND APPARATUS FOR STRESS-STRAIN TESTING Qct. 7,1952 3 Sheets-Sheet 1 Original Filed June 3, 1944 CLARENCE M- Z ENER,IIELBERTM/NWNK LE,

06f. 7, 1952 Y C, M, ZENER ET AL 2,612,774

METHOD AND APPARATUS FOR STRESS-STRAIN TESTING Original Filed June 5,1944 I5 Sheets-Sheet 2 DI AM ETEF?.

SWW/nimm l l ELARENEENLZENER,

DELEIERTMVNWNKLE,

ILOAD E LONC-BATION Oct. 7, 1952 c. M. zENx-:R ET AL 2,512,774

METHOD AND APPARATUS FOR sTREss sTRATN TESTING 3 Sheets-Sheet 5 OriginalFiled June 5, 1944 vwc/nto'w, E LARENCEMA'ZENER, DELBERT MNWNKLE,

MEG.

,m M d m mm f @rented Oct. 7, 1952 MErHon AND APPARATUS For. smesssTnAINTESTING l Clarence M. Zener, Newton Center, and Delbert Van Winkle,Newtonville, Mass.

original applicati@ June A3, 1944, serial No. 538,674. Dividedand thisapplication August 30,194.5, SerialNo. 613,686 f fromm-s. (o1. 'i3-89)(Granted under the act of March 3, r1883, as

amendedApril 30, 1928;4 370 v( G. 757) The invention described hereinmaybe manufactured and used by `or for the Government; for governmentalpurposes, without the payment to us of any royalty thereon.

This application is a division of our copendi-ng application Serial No.538,674, led June 3, 1944, which matured into Patent No. 2,423,867onJuly In many mechanical constructions, and particularly in ordnance,metallic components are subjected in servicerto deformations., both inthe elastic and plastic range, at rates of strain far in excess of thosepermitting obtainingof` stressstrain readings in conventional .manner ona standard tensile testing machine. To obtain the stress-strainedproperties of metals under such severe conditions by subjecting them .to.stresses in a high speed testing machine presents unusual problems ofmeasuring and recording the rapidly varying stress and strain in thetest specimen.

Accordingly, it is the object of this invention to provide an improvedtesting `and recording method and apparatus that will automaticallyrecord precisely equivalent values of stress and strain in a specimensubjected to the application of stress at a rapid rate.v

The specic nature of the invention as well as other objects andadvantages thereof Will clearly appear from a description of aprerferredembodiment as shown in the accompanying drawings in which: v I

Fig. 1 is aschematicdiagram-of the `indicating and recording circuitutilized.

Fig. 2 is a top kelevational view of one Vform of resistance strain gageoperating on reduction in diameter of the test specimen. H

Fig. 2A is a sectional Viewjtaken along the plane EaP-2410i Fig.2.

Fig. 2B is a sectional view taken along .the plane 2b- 2b of Fig. 2. v

Fig. 2C is a side elevational view partly in longitudinal section ofanother form of Vresistance strain gage wherein such gage is soconstructed as to produce an indication proportional k.tothe elongationof a test'specimen. l

Fig.'2D is an enlarged perspective view of a part utilized with thestrain gage illustrated in Fig. 2C.

Fig. 3 is a side elevational'view of an attachf ment utilized to providean indication of the stress exerted by a standard testingmachine onatest specimen. K y

Fig. 3A is a top elevational View of .Fig..3.

, 2 A gagein Fig. 3 -attached to a standardtesting machine.

Fig. 5 is a schematic view of a conventional- Wheatstone circuit whichis incorporated in the circuit of Figi.

Fig. 6 illustrates 'the film record obtained `with vare known. fResistance type-electriclstraingages are attached .to the 'test specimenin such a man'- ner that the strain produced in the Atest specimen isquantitatively `transmitted. tothe electric strain gage. Likewise Ibymeans vof an attaclnnent to theyst'andard testing machine, a `strainproportional to the Vstress .applied .to .the test specimen by theytesting machine is applied kto an velectric strainy gage. Lhe.resistance elements of these strain gages are each incorporated in aseparate y alternating .current Wheatstone bridge circuitA so that .theresistance change of the gages `,is translated into a varying potentialappearing across the output terminals of vthebridge circuit. The outputpotentialof each bridge is 'thenamplified rectied, passedthroughasuitable meterand applied to theplates of a cathode'ray oscillogr'aphhaving .a camera portion `associated ,thereWithL The electronbeam of the.o scillograph is focused tolproduce apoint source of light `on thecathode ray screen. With the voltage derived fromv the stress .measuringlgage -applied .to` the vertical plates and ,thefvoltages derived fromthe strain measuring "gage appliedto the `horizontal iIilltes vcfthecathodefray oscillograph, the electrdn'bca'm? Will.describea-curvefcorrelatedto stress-,strain conditions in .the test' specimen',as the ,testpro'- ceeds. Thfsgpathia of course, simultaneouslyphotographed bythe camera portionofjthe Ao ,s-

lcillographV torprovide a permanent record'. They meter .is utilizedfasan indicator duringcalibration of the apparatus and may'yalsobereadduring low vspeed tests to provide ,aiia,..itignalrecord'of,thetest` Theiniproved .stress andstrainggages utilized Y ,maistesting :apparatus ,sen/eas .csensiczlersif Fig. 4isaperspectivej-tlewfshowing ,tnegstran v55 A.iQ c.. lthey .trasicrma-.strain displacement or 3 about .25" for example into approximately a.003 strain in the electric strain gage. Such desensitizing is necessaryin order that an electric resistance type strain gage may be employedand thereby permit the measurement of stress or strain by an electricalprocess.

In Fig. 4 there is disclosed an attachment for a standard testingmachine whereby the load exerted by such machine on a test specimen isconverted into a proportional change in resistance of an electricresistance strain gage. The attachment comprises an L-shaped supportingblock a horizontal arm 2 which is secured to a portion 20 of the frameof a standard tensile testing machine (not shown) by means of a clampingbar 4 mounted on a screw 5 and clamped against frame portion 20 by meansof a nut 6. The vertical arm 3 of support I lies adjacent to the end ofthe balance arm 2| of the testing machine. On top of vertical arm 3 acantilever beam element 1 is mounted by means of a plate 8 and screws 9.On the end of cantilever beam element 1 there is provided a contactmember which preferably comprises a screw threaded through beam element1 and a lock nut The end of screw I0 engages the top surface of balancearm 2| when arm 2| is in its position of balance.

The proper proportions of the cantilever beam 1 will obviously vary withthe maximum force which the balance arm of the tensile machine iscapable of exerting. However, the determination of the size of the beamcan be made from the standard formulas by computing the dimension of abeam that will yield a small displacement, on the order of a quarter ofan inch, under the upward force exerted by the balance arm at themaximum load to be used and which in addition will yield such an end.displacement with a surface strain preferably less than .003, as a .003strain is currently considered a reasonable upper limit for resistancestrain gages. On both the top and bottom surfaces of cantilever beamelement 1 conventional electric resistance strain gage elements RI andR2 are fastened, preferably by cementing. As stress is applied to thetest specimen by the testing machine, the balance arm 2| will tend torise in conventional manner and hence will produce a deflection ofcantilever beam element 1. The surface strain produced in beam element 1produces in conventional manner, a change in value of electricvresistance of strain gages Rl and R2.

One embodiment of the gage utilized for measuring the strain in aspecimen 30 is shown in Fig. 2., This gage comprises a tempered springsteel frame 40 which is formed into approximately a figure 8. A pair ofreinforcing plates 4| are secured, preferably by welding, in opposedrelationship to the frame 40 at its narrowcentral region. Both of theplates 4| are drilled and tapped to receive yokes 42 and a roller 43 ismounted in each yoke 42. Lock nuts 44 are threaded on the end portion ofyokes 42 extending through reinforcing plates 4|.Electric-resistancestrain members R3 and R4 are cemented to the loops ofthe frame 40 and suitable leads 45 are provided to connect theresistance strain members R3 and R4 to the indicating circuit which willbe described. The frame 40 is so'proportioned that when mounted around atest specimen 30 of conventional shape, the rollers 43 will yieldinglyengage a reduced diameter portion 3| of a test specimen 30. As the testspecimen 30 is subjected to stress in the standard ltensile testingmachine, its diameter will be reduced and it will begin to neck down atthe reduced cross section 3|. Hence the rollers 43 tend to move togetherdue to the spring force of the frame 40. Accordingly, the resistance ofthe electric resistance strain gages R3 and R4 is proportionallychanged.

The rollers 43 perform an important function in that they automaticallylocate themselves throughout a test at the smallest diameter portion ofthe test specimen 30. It is of course impossible to predict prior to thetest at just what point along reduced diameter portion 3| of the testspecimen 30 the greatest necking down will occur but a gage constructedin accordance with this invention automatically locates itself at thenarrowest portion and thereby provides a more accurate indication of themaximum strain produced in the test specimen.

In Fig. 2C there is shown another form of gage for indicating the strainin a test specimen, this gage being constructed to produce an indicationproportional to the elongation of a test specimen. This gage comprises aspring tempered frame 50 shaped in a form approximating a figure 8. Apair of collars 5| 'are secured to the frame 50. preferably by welding,in opposed relationship at the narrow central portion of the frame 50.Electric resistance strain gages R3 and R4 are respectively cemented toeach of the loops of the frame 50. Suitable leads 45 are provided toconnect resistance gages R3 and R4 to the indicating circuit which willbe described.

The test specimen 32 utilized in conjunction l with this gage is ofsomewhat different configuration. Test specimen 32 has customarythreaded ends 33 for mounting in a standard tensile testing machine andan intermediate reduced diameter Vportion 34. The juncture between thecentral portion 34 and end portions 33 forms shoulders 35. The gage isassembled by inserting specimen 32 through the collars 5|. The collars5| are then pressed together, thus stressing frame 50,'and keys 53 arerespectively inserted between Acollars 5I and shoulders 35 of the testspecimen. The keys 53 have a raised portion 54 which fits in the bore ofthe collars 5|. The test specimen 32 is then mounted in a standardtesting machine and a tensile stress applied to the specimen. As thespecimen elongates under such stress the collars 5| separate, followingthe elongation. The strain in frame 50 is thereby reduced proportionallywith the elongation of specimen 32. Hence the resistance of theresistance gages R3 and R4 changes in proportion to the strain in thetest specimen. It is of course obvious that the frame 50 must be undersuicient strain in the original assembled position so that such strainwill not be wholly relieved at the point at which the rupture ofspecimen 32 occurs.

It should be noted that both types of strain gages herein describedremain on the test specimen throughout the test andare unharmed byfracture of the specimen. The useful life of such gages is thuspractically unlimited.

In Fig. 1 there is shown a schematic diagram of the indicating andrecording circuit utilized in conjunction with the apparatus heretoforedescribed. The resistance elements RI and R2 of the stress measuringgage shown in Fig. 4 are incorporated in two arms of a conventionalWheatstone bridge circuit indicated as the stress bridge 6|. It shouldbe noted that by utilizing two active stress gages in the two .variablearms of the bridge circuit, the effect of temperature is substantiallycancelled out between the two arms and hence onlychanges in resistancedue to strain are measured. The resistance elements R3 and R4 of eitherthe strain gage shown in Fig. 2 or Fig. 2C are incorporated as two armsof a conventional Wheatstone bridge circuit indicated as the strainbridge 62. An alternating input potential is supplied to both bridges bya common oscillator 60.

The bridgel circuits utilized for both the stress bridge 6l and strainbridge 62 are substantially identical and are shown in detail in Fig. 5.The resistance elements of electric resistance strain gages Rl and R2are connected as two variableV arms of the stress bridgeV whilefixed-resistances R6 and- R1 form theffixed arms of the bridge. Theresistance elements RI, R2, R6 and R1 are preferably substantially'equal under zero load conditions, thereby increasing the sensitivity ofthe bridge circuit. Since as indicated in Fig. l the oscillator 66supplies both the'stress bridge 6l and the strain bridge 62, one side ofthe oscillatoris grounded to prevent coupling between the two bridgecircuits. 'Ihe output of the bridge circuit is connected throughv animpedance matching transformer T. which as willbe seen, permits thegrounding of the subsequent amplifier stage. To vary the resistance ofthe xed arms of the bridge, a variable resistance R5 and a variablecondenser Cl are connected to switches Sl and S2 in the manner indicatedin Fig. 5 whereby either resistance R5 or condenser CI can be connectedin parallel with fixed resistances R6 and R1. The bridge may vthus bereadily balanced. Strain bridge 62 constitutes a similar arrangementonly utilizing resistance elements .R3 and Rd of the strain gages as thevariable arms.

Referring again to the schematic diagram in Fig. l, the outputs ofstress bridge 6| and strain bridge 62 are respectively connected throughswitches 63 and 64 to the input of variable gain linear amplifiers 65and 66. A voltage divider 61, such as a potentiometer, isinterconnected" with switches G3 and 64 in such a manner thatmanipulation of switch 63 will connect potentiometer 61 between stressbridge 6l and amplier 65 and manipulation of switch 64 will connect thepotentiometer 61 between strain bridge 62v and amplifier 66.Potentiometer 61 is` utilized for calibration purposes as will bedescribed. g

The outputs of amplifiers 65 and 66 areconnected respectively torectifiers 68 and B9 and milliammeters 16 and 1|. Switches 12 and 13 arerespectively connected across the outputs of amplifiers 65 and 66 andmaybe operated to connect an oscillograph 14 across the output of eitherof the amplifiers.

Connected to the outputs of milliammeters11 and 1| are. double pole,double throw switches i5 and 16V respectively. In one position, switch15 connects its channel to the vertically deecting plates 11 of acamera-type cathoderay. oscillograph 86; in the other position, switch15 connects vertical plates 11 to a calibration sweep circuit unit 16.Switch 16 performs a similar function for the strain bridge channel, inone position connecting its channel to horizontally deiiecting plates 18of the cathode ray oscillograph and in the other position connectinghorizontal plates 1S to the calibration sweep circuit. unit 19.

Calibration sweep circuit unit is shown in detail in Fig. i8 andcomprises. an arrangement for charging or discharging a condenser C2through a resistance R8. A battery B is arranged to` be connectedv by aswitch S3 to charge the condenser C2 through resista-neemt; AlresistanceR9 is connected across the battery" and vswitch whereupon opening ofswitch S3`permitstheicondenser C2 to ldischarge through'. theresistances R8 and R9 inlseries. The voltage across condenser C2 isapplied to either thefvertical plates 11 `or the horizontal platesI18.01 the cathode ray oscillograph by means of the switching units 15and 16. It should be noted. thata trace of `very uniform intensity isobtainedby closingv and opening switch S3, as `this-causes' ia traverseandretraverse of 'the -cathode ray beam. `The sum of the velocities oftraverse'andi'retraverse'of the beam at any one point issubstantiallyconstant.

The: oscillograph 14fis` utilized for a balancing indicatorwfor'eitherAth'estressbridge 6| or the strain bridge 62. J The switch 12 may bethrown to connect the oscillographl 14 :across the output ofsamplier 65vand stress bridge 6| .may then be balanced to produce zero indicationon oscillograph 14 by means of variable resistance R5 and variablecondenser Cl of the bridge circuit. In similar manner by operation 'ofswitch 13 the oscillograph 14 may be utilized to balance the strainnbridge 62. A sensitive meter couldof course be substitutedforoscillograph 14. With theapparatus describedcalibration lines onthetscreen of the'cathode ray oscillograph Si! maybe readily produced.Itis, of course, understood that the light traces producedon the cathoderayscreen are ysimultaneously photographed by the camera portionassociated therewith, hence the calibration lines lare first produced ona particular iilm and then the stress-strain curves of a particularspecimen are photographed on the same film. To produce horizontalcalibration lines corresponding to units of applied stress, the stressbridge 6| is iirst balanced vwith zero loadon the stressgage'illustrated .iny Fig. 3. Switch 63 is then operated to connectpotentiometer Y61 between stress bridge 6| and amplier 65. A known loadgreater than the maximum expected load in the test to be run is thenapplied to the stress gage. Such, load produces an unbalance of stressbridge 6|' and hence a voltage output proportional to the load. Thepotentiometer 61 is preferably divided into tenth units and hencesuccessive voltages of .1, .2, .3, etc., of the maximum loadfvcltage mayb e applied to the input of variable gain amplifier 65. The gain ofamplifier 65 is adjusted sothat the maximum voltage applied produces adeiiection on milliammeter 10 corresponding to a full verticaldeiiection of the beam ofthe cathode ray oscillograph 6i). Switch 16 isoperated to connect the horizontal plates 18 of the cathode rayoscillograph to the calibration sweep circuit unit 19. Then foreachvoltage step obtained from potentiometer 61' the switch S3 ofcalibration sweep circuit unitv 19 is manipulated, which' causes thecathode ray beam to traverse a horizontal path between horizontal plates18. The vertical position ofthe horizontal path ofthe beamis, lofcourse, determined by the voltage on the vertical plates 11,whichvoltage is in turn determined by the setting of -potentiometer 61.Thus a seriesjof horizontal lines corresponding to one tenth units ofmaximum stress are traced across the cathode 4ray screen and'whenphotographed by the camera associated therewith appear as in Figs. 6 and7.

The vertical calibration lines are obtained in an `exactly similarmanner by applying a known strainv to theresistance elements R3 and R4of the strain gages shown in Figs. 2 and 2C. Switch 64 of'courseisoperated tocan-neet potentiometer y6l between strain bridge'62'v and amplifier 66.

Switch '1B is` positioned to connect the strain bridge channel to .thehorizontal plates 18 of the cathode ray oscillograph 80 While switch l5is operated to connectv the sweep circuit calibration unit 19 to thevertical plates 11. l In this manner a series of vertical lines aretraced on the cathode ray screen equivalent to one tenth units ofstrain. The appearance of.` such calibration lines on the lm is shown inFigs.6 and 7. 4-

It will be appreciated that such a method of calibration eliminates anynon-linearity which might exist in the rectiiier, the ampliiier or thecathode ray deflection.. Since the first step in obtaining eachstress-strain record,.consists in placing the calibrated coordinatesystem upon the nlm, it will be apparent that a high degree of accuracyis consistently obtained.

To make the actual recording of the'stressstrain curve of a specimensubjected to stress -in a standard tensile testing machine, the stressgage shown in Fig. 3 is applied to the tensile machine in a mannerheretofore described. The stress bridge 6| is then balanced to arzeroloading. A load slightly over the maximum expected load is then producedin the testing machine. This of course produces a deflection of thestress gage and in turn a proportional unbalancing of the stress bridge6|. The gain of amplifier 55 is then adjusted so that the milliammeter'l0 reads a value corresponding to that .necessary to produce fullvertical deflection of the cathode ray beam.

The strain gage illustrated in Fig.. 2. is then placed on a cylindricalrod Whose diameter. is equal to or slightly larger than theinitialdiameter of the testspecimen.. The strain bridge. 62 is thenbalanced undertheseA conditions. The strain gage is then placed. onv acylindrical rod whose diameter is slightly smaller than the smallestdiameter anticipated to be. produced in the test specimen and the gainfof the amplifier 66 is adjusted until the milliammeter 'H reads a valueequivalent to that which will produce fulll horizontal deflection of thecathode ray beam. A similar calibrating method is applied when utilizinga gage of the type shownin.Fig.12C.3

The apparatus is then in condition fortesting of a specimen. The straingage is placed on the test specimen y30, orV 32 and the test specimeninserted in the standard tensile testing machine. The machine is setinto operation and stress may be-appliedv at any desired ,rate`depending only upon the characteristics of the particular testingmachine, As the test proceeds, amplified voltages proportional tothestress applied. to the.v test specimen are produced upon thevertically deiiecting plates 'Il of oscillograph BD and amplied voltagesproportional to strain in thel test specimen are produced on thehorizontally deflecting plates 18. The cathode ray beam thus Vtraces aload deformation curve on ,the cathode rayscreen and this tracing isrecorded upon the film Vinthe camera associated therewith whichiilm,already has the calibrated coordinate system uponit. Since the apparatusrecords instantaneous diameters of Vthe test specimen, atimestress-strain curve may be computed, evenA throughout necking of thespecimen. kIt is therefore' apparent that accurate recording of allportions of the stress-strain curve is obtainable independent of therate of application of stress of the machine. Figures Sand '7 illustratethe nlm records obtained respectively with strain gages as in Figs. 2and2C. l.

This-apparatus is particularly` useful even in testswherethe stressisapplied'at'a slow. rate in 8 thatit will accurately record thosecritical portions Vof Vthe stress-strain curve, for example, around theyield point where sudden changes in theelongation and stress values areencountered. With the standard apparatus it is practically impossible toobtain an accurate measurement of the stress in this region by thecustomary manually operated balancing arrangement.

We claim: v l. lIn a system for. graphically indicating theinstantaneous stress-strain characteristics of a test specimen duringloading thereof, the combination of electrical means responsive to thestress produced by loading a test specimen. electrical means responsiveto the resulting strain in the test specimen, means for generatingvoltages in said stress and strain responsive means respectivelyproportional to known values of stressv and strain, means for dividingeach of said voltagesv into a plurality of predetermined consecutiveparts, means for amplifying said voltage divisions, a, cathode rayoscillograph including a viewing screen for indicating thereon theposition of an electron beamA and a camera portion for simultaneouslyrecording the deflection of said beam, a sweep circuit for moving saidelectron beam horizontally or vertically across said screen, means forsuccessively applying said voltage divisions to said oscillograph toproduce a plurality of intersecting horizontal and vertical traces onsaid screenfor recording by said camera portion, said vtracescorresponding respectively to known values .of stress and strain, switchmeans for rendering said voltage dividing means and said sweep circuitinoperative during loading of the test specimen, means for loading thetest specimen .up to and beyond the point of rupture thereof wherebycorresponding voltage changes are respectively produced by said stressand strain responsive means, and means for combining at right angles theeffects of said voltage changes to deflect said electron beam so as toproduce the stress-strain curve of the test specimen, said oscillographcamera portion being arranged to superimpose said stress-strain curve onsaid coordinate record of said horizontal and vertical traces.

2. In a system for graphically indicating the instantaneousstress-strain characteristics of a test specimen during loading thereof.the combination ofa firstl alternating current bridge circuit forgenerating voltages proportional to the stress. applied to the testspecimen, a second alternating current bridge circuit for generatingvoltages proportional to the resulting strain incurred in the testspecimen, means for unbalancing in turnsaid rst and second bridgecircuits to generatevoltages corresponding respectively to known valuesof stress and strain, potentiometer means for dividing said unbalancevoltages into a plurality of predetermined equal parts, means foramplifying and rectifying said voltage divisions,'a cathode rayoscillograph including a viewing screen for indicating thereon theposition of an electron beam and a camera portion for' simultaneouslyrecording the deflection of said beam, a sweep circuit including acondenser and a source of power, means for charging and discharging said.condenser for each setting of said potentiometer means for producing aplurality of consecutive horizontal and vertical traces on saidoscillograph screen for recording by said camera portion, said tracescorresponding respectively to 'known values of stress and strain,switchmeans for'disconnecting said potentiometer means rand said sweepcircuit from said `,ridge circuits and` said cathode rayxoscillographduring loading of the test specimen, and means for loading the testspecimen up to and beyond thepoint of rupture thereof whereby theeffects of the voltages respectively generated by the correspondingunbalance of said stress and strain bridge circuits are combined atright angles to deflect said electron beam so as to produce thestress-strain curve of the test specimen, said oscillograph cameraportion being arranged to superimpose said stress-strain curve on saidcoordinate record of said horizontal and vertical traces.

31A method of Calibrating a stress-strain curve obtained in astress-strain testing apparatus including a stress responsive means, astrain responsive means, a cathode ray tube having vertical andhorizontal deflection means, two

l voltages applied through one of said channels and corresponding toknown increments of stress, similarly applying a sweep voltage to theother deflection means and recording a second series of traces on saidscreen at right angles to the nrst series produced by voltages appliedthrough the other of said channels and corresponding to known incrementsof strain, and thereafter recording the trace on said screen produced byvoltages applied through both channelsand corresponding to thestress-strain curve of the specimen under test on the previouslyrecorded coordinate system so as to nullify any deviations from the truevalues of the stress-strain curve which may be produced as a result ofany nonlinearity inherent in the apparatus.

4. A method for nullifying the non-linearity inherent in the electricalcircuits of an apparatus for indicating the simultaneous stressstrainconditions produced in a test specimen under load, said apparatusincluding stress responsive means, strain responsive means, a cathoderay tube having vertical and horizontal deilection means, and twoamplifier channels connecting said stress responsive means to saidvertical deection means and said strain responsweep voltage to one ofsaid deection means and applying voltages corresponding to knownincrements of stress to the other of said deection means through one ofsaid channels, similarly applying the sweepvoltage to the other of saiddeflection means and applying voltages corresponding to known incrementsof strain to the one of said deection means through the other of saidchannels, recording-the horizontal and vertical traces thus produced toform a visual coordinate system, applying a load to the test specimensuicient to effect elastic and plastic deformation thereof, applyingvoltages resulting from the simultaneously changing stress and strainconditions of the test specimen to the one and the other of saiddeflection means respectively through both channels,'and superimposingthe resulting stress-strain curve on the previously recorded coordinatesystem so that the stress and strain values at any point on the curveare indicated by the known values of the vertical and horizontal traces.

' CLARENCE M. ZENER.

DELBERT VAN WINKLE.

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